When an upgrade is required for a dam with an inadequate spillway system, the potential impact of tailwater during an extreme storm in an inundation study should be considered. This paper provides a practical case as an example, it investigates the impacts of significantly uncertain tailwater elevations on the dam breach and the resulting flood magnitudes. The analysis of the tailwater uncertainty has limitations when applying the available computer programs with the breach analysis. This study was conducted by performing a sensitivity analysis of critical breach outflows and tailwater effects on the dam. The results may be used to review alternatives for upgrading spillways to minimum required spillway criteria.